Introduction to Photochemistry

Abstract

This chapter summarizes some general concepts in photochemistry, with two aims: to provide an overview of phenomena and empirical rules that will be discussed on theoretical grounds in the next chapters, and to present a language and some physical laws concerning light and its interaction with matter. We shall introduce the main differences between thermal chemistry that takes place in the ground electronic state, and photochemistry, that involves optical excitation. The first overview of elementary photoinduced events will highlight the different ways the excitation energy can be disposed of. We shall distinguish primary and secondary processes and define their quantum yields. Excitation rates will be introduced in connection with the Lambert and Beer law. Some elementary examples of photochemical kinetics will be discussed. Much more extended introductions to photochemistry can be found in well-renowned textbooks by Wayne (Principles and Applications of Photochemistry. Oxford University Press, Oxford, 1988 [1]), Wardle (Principles and Applications of Photochemistry. Wiley, Chichester, 2009 [2]), Turro (Modern Molecular Photochemistry of Organic Molecules. University Science Books, Sausalito, 2010 [3]), Balzani (Photochemistry and Photophysics: Concepts Research, Applications. Wiley, Chichester, 2014 [4]), Rohatgi-Muckerjee (Fundamentals of Photochemistry. New Age International, New Delhi, 2017 [5]), and others.

Keywords

Problems

1.1

Compute the energy, in kJ/mol, of an infrared photon with frequency 1500 \({\mathrm{cm}}^{-1}\) and of a visible photon with wavelength 500 nm.

1.2

Compare the irradiance of sunlight in the visible part of the spectrum (about 500 W/m\(^2\) in a clear day) with: (1), the irradiance of a 1 mW He–Ne laser with a beam diameter of 1 mm and, (2) the irradiance of a 100 W tungsten lamp at a distance of 2 meters, assuming that 2% of its power is converted into visible light.

1.3

A molecule is exposed to monochromatic light with \(\lambda = 500\) nm and irradiance 100 W/m\(^2\). How frequently does it absorb a photon, if its molar absorption coefficient at 500 nm is \(\varepsilon =1000\) mol\(^{-1}\) L cm\(^{-1}\)?

1.4

The chlorofluorocarbons are photodissociated in the stratosphere only when they reach an altitude where UV light with sufficiently short wavelengths is present. CCl\(_3\)CF\(_3\) does not absorb significantly at \(\lambda > 220\) nm. The C-Cl dissociation energy is about 330 kJ/mol. Are short wavelengths needed for the photodissociation because of the absorption spectrum of CCl\(_3\)CF\(_3\) or because of the C-Cl bond strength?

1.5

Compute the lifetimes of \(T_1\) and \(S_1\) of benzophenone and its phosphorescence quantum yield. The same for naphthalene, with the addition of its fluorescence quantum yield. Use the lifetime of each process indicated in Fig. 1.2 (remember that the inverse of a lifetime is the rate constant of the process).

1.6

In the example of photochromic reaction kinetics discussed in Sect. 1.6.3, suppose the experiment with high irradiance is repeated at a higher temperature, so that \(K_{A\rightarrow B}\) increases by 80% and \(K_{B\rightarrow A}\) by 50%. Based on the data that can be inferred from Fig. 1.5, which fraction of B is expected at 150 s?